Question

Assertion (A): Fluorine has smaller negative electron gain enthalpy than chlorine
Reason (R): The electron-electron repulsion is higher in chlorine than in fluorine

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A).
• C. (A) is correct but (R) is incorrect.
• D. (A) is incorrect but (R) is correct.

Hint:

Fluorine is an exception in electron gain enthalpy trends because its very small size causes strong electron-electron repulsion in the compact \(2p\) orbital.

Answer 1

The Correct Option is C

Step 1: State the assertion and reason.
Assertion (A): Fluorine has a smaller negative electron gain enthalpy than chlorine. Reason (R): The electron-electron repulsion is higher in chlorine than in fluorine.
Step 2: Recall what electron gain enthalpy (EGE) means.
Electron gain enthalpy is the energy change when a neutral atom in the gas phase accepts an electron to form an anion. A more negative value means the process releases more energy and the atom has greater electron affinity.
Step 3: Compare EGE of F and Cl.
EGE of F $\approx -328$ kJ/mol. EGE of Cl $\approx -349$ kJ/mol. So Cl has a more negative EGE than F. This means F has a smaller negative EGE than Cl, which makes Assertion A CORRECT.
Step 4: Explain WHY fluorine has smaller negative EGE.
Fluorine is very small (2p orbital is compact). When an incoming electron enters the small 2p orbital, it experiences strong electron-electron repulsion from the existing 7 electrons already occupying the compact 2p subshell. This repulsion in F reduces the energy released, lowering the magnitude of EGE.
Step 5: Evaluate the Reason (R).
The Reason states that electron-electron repulsion is higher in CHLORINE than in fluorine. This is INCORRECT. In reality, repulsion is higher in fluorine because of its smaller, more compact 2p orbital. Chlorine's 3p orbital is larger and less congested, so incoming electrons experience less repulsion.
Step 6: Conclude on the assertion-reason pair.
A is correct (F has smaller negative EGE than Cl), R is incorrect (repulsion is higher in F, not Cl). This corresponds to option 3.
\[ \boxed{\text{A is correct, R is incorrect (Option 3)}} \]